Xenotransplantation offers the best near-term hope for satisfying the critical limitation imposed on the field of transplantation today by the severe shortage of cadaveric allogeneic organs. Our studies in the previous project period of this Program Project grant have made considerable progress, and organ survivals of both heart and kidney xenotransplants from pigs to baboons are now measured in months rather than days. A major factor in this improved survival has been the development of a new, knock-out strain of miniature swine (GalT-KO), which do not express the Gal epitope on their cells, thus avoiding the severe rejection previously caused by natural anti-Gal antibodies. We have also shown that the greatest improvement in organ survivals using these new donors is observed using protocols designed to induce tolerance. Nevertheless, additional improvements will be required before this technology is ready for clinical application. This renewal application brings together five projects devoted to the most promising approaches and the most pressing problems in this field of research: 1) tolerance induction through vascularized thymic transplantation;2) tolerance induction through mixed chimerism;3) modeling of tolerance in mice with human immune systems;4) Viral pathogenesis in Xenotransplantation;and 5) thromboregulatory barriers to Xenotransplantation. All of these projects-are highly interactive, and utilize numerous shared resources, many of which will be made available through the large animal core. The work will be carried out in a unique environment, which provides interactions with scientists working in basic and cellular immunology and working on relevant small and large animal models in the same research center, as well as with clinicians committed to taking new therapies to clinical applications.